ORM: Change parent's "updated_at" timestamps when model gets updated in FuelPHP

Question

Let's say I have a Model_Post that has_may Model_Comment.

Is there any way that the updated_at field of Model_Post gets updated every time that a new comment is saved? I checked the documentation for the observers, but I cannot find how to do that in a "ORM way".

This code doesn't seem to work:

class Model_Comment extends \Orm\Model
{
    protected static $_properties = array(
        'id',
        'post_id',
        'text',

    );
    protected static $_belongs_to = array('post');
    protected static $_observers  = array(
        'Orm\\Observer_UpdatedAt' => array(
            'events'          => array('before_save'),
            'relations'       => array('post')
        )
    );
}

class Model_Post extends Model
{

    protected static $_properties = array(
        'id',
        'title',
        'text',
        'updated_at'
    );
    protected static $_has_many   = array('comments');
}

I create a new comment using this code:

$comment             = Model_Comment::forge();
$comment->message_id = Input::post('message_id');
$comment->text       = Input::post('text');
$comment->save();

Answer 1

Finally I found the way to do that thanks to Uru's comments.

This code works now as I wanted; it was simpler than I thought.

Models:

class Model_Comment extends \Orm\Model
{
    protected static $_properties = array(
        'id',
        'post_id',
        'text',
    );
    protected static $_belongs_to = array('post');
}

class Model_Post extends \Orm\Model
{
    protected static $_properties = array(
        'id',
        'title',
        'text',
        'updated_at'
    );
    protected static $_has_many  = array('comments');
    protected static $_observers = array('Orm\Observer_UpdatedAt');
 }

And in the controller:

$comment       = Model_Comment::forge();
$comment->text = Input::post('text');
$parent_post   = Model_Message::find(Input::post('post_id'));

$parent_post->comments[] = $comment;
$parent_post->save();